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The only way you would see a decrease in engine RPM is if you are changing gears or if you have a CVT transmission. I would assume you have an auto trans bolted to this engine? Most Autos have a 1:1 ratio in thier "3rd" gear with an overdrive of around .8:1 If you are not bouncing between gears then your engine speed will not change regardless of how aerodynamic your car is at a set speed. NOw the advantage to you getting the 4Runner more aerodynamic is that you can reduce your rear axle gearing as it will take less HP to move it at a set RPM. So lets say you make 120 HP at 3000rpms (diesel) and currently you are traveling at 65 mph at 3000rpms...if you make your car more efficent at speed you may only need 80 hp to maintain 65 MPH so you could re-gear your truck to 4.11 or 3.73 and drop your engine rpm down when cruising at 65 MPH. Hopefully I explained this alright.
Here's a calculator that is really fun to play with. You can enter in your gear ratio and your automatics final 'overdrive' ratio and then see how a gear swap will decrease your engine rpm at a given speed.
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